In the past decade, environmental considerations have forced reevaluation of many established products. Contact adhesives based on elastomeric polymers that have been available commercially for several decades, fall into this category.
The term contact adhesive, for our purposes, refers to an adhesive which when coated on two substrates adheres to itself upon substrate contact and drying. These products have been formulated in both solvent and water-based systems. While such products have been used in both consumer and industrial applications, generally the use of solvent based systems is widespread. This is due to their consistent performance in various bonding applications under a variety of operating conditions.
Water-based systems on the other hand are generally sensitive to operating conditions. As a result, their bonding properties fluctuate. For these reasons, water-based products have been used sparingly, especially in semi-production type operations.
Many of the commercially available water-based contact adhesives are based on polychloroprene latex and acrylic/vinyl acrylic type emulsions.
The stability of polychloroprene latices in combination with other commonly used ingredients necessary in adhesive compositions is a general problem. In recent years, latex compositions with improved stability have become available. These are generally based on polychloroprene-methacrylic acid copolymers stabilized with partially hydrolysed polyvinyl alcohol.
Another common characteristic of adhesive formulations based on polychloroprene latex is that they require the use of antioxidants and acid acceptors. Generally, these products have to be used in a dispersed form which has the overall effect of reducing the stability of the formulation and increasing its cost. An example of this is the use of zinc oxide which is often used in a dispersed form in polychloroprene based formulations. The zinc oxide serves two purposes: one to act as an acid acceptor; and secondly to serve as a crosslinking agent with carboxyl functional groups.
Formulations based on acrylic and vinyl acrylic emulsions generally display long "open times" and a characteristic best described as a permanent "dry tack". While long open times may be a desirable feature in a few applications, they are a major short coming in many production type applications of contact adhesives. Likewise, "dry tack" is desirable in pressure sensitive applications. Many contact adhesive uses, such as for decorative laminate assemblies, leather goods production, etc. require permanent bonds.
A novel way to combine the elastomeric features of a polychloroprene latex with the toughness of a polyvinyl acetate based emulsion has now been discovered.